Process for Producing Resin-Coated Pigment, Resin-Coated Pigment, Aqueous Pigment Dispersion and Aqueous Colorant Composition Containing the Same

ABSTRACT

A process for producing a resin-coated pigment capable of providing an ink and a paint being superior in dispersion stability of a pigment and having rapid drying property, high gloss and density without containing those such as an organic solvent and a salt that cause the lowering of performance of pigment dispersion, characterized in that a pigment is dispersed in an aqueous solution in which an anionic group-containing resin is dissolved in the presence of a basic compound and then, the anionic group-containing resin is precipitated on the surface of the pigment by removing the basic compound in the aqueous solution, using an ion exchange means.

TECHNICAL FIELD

The present invention relates to a process for producing a resin-coatedpigment, a resin-coated pigment, an aqueous pigment dispersion and anaqueous colorant composition containing the same.

BACKGROUND ART

As a colorant for various inks and paints, dyes are used in a fieldrequiring higher level of sharpness and printing or coating property andon the other hand, pigments are used for a field requiring waterresistance and light resistance, and in particular, the tendency isremarkable in the field of an ink for ink jet recording. A conventionalink jet recording system has been developed from the viewpoint that themaximum problem to be solved is how superior image sharpness and stablespouting property can be imparted to ink. However, the number ofprevailed digital cameras has been greatly increased recently because ofeasy handling and high image quality and the target of ink jet recordingextends to the field of silver salt photography. Accordingly, theimpartation of light resistance is going to be a problem on par with thesharpness so that color is not faded nevertheless long preservation.Therefore, conventional dyes are going to be rapidly converted topigments as the colorant of ink, but there remain many problems thatmust be solved when pigments are utilized.

For example, since a dye is dissolved in dispersion medium in molecularstate, it affects little flowability and re-dissolving propertydominating the spouting property of the ink even after elapsed time or aconcentrated state and can keep high spouting stability. However, apigment existing in particle state easily causes phenomena such ascoagulation, sedimentation and solidification, and such phenomena lowerflowability and re-dissolving property and as a result, the spoutingstability is deteriorated. In particular, an organic pigment superior insharpness must be dispersed as fine particles in aqueous medium in theink jet recording system requiring high sharpness, whereby the area ofhydrophobic surface is increased to inevitably cause coagulation.

For dispersing a pigment in an aqueous medium, methods for wetting thehydrophobic surface of the pigment using a dispersant and stablydispersing it in the aqueous medium are usually used. Among them, amethod of adsorbing a surfactant or a polymer compound on the surface ofa pigment is widely used. However, a strong adsorption force is requiredfor keeping the dispersion state of the pigment and there are littledispersants exhibiting good pigment dispersibility for a long period.Further, since a large quantity of a dispersant is necessary forimproving the dispersibility of a pigment by the method, thedisengagement speed of the aqueous medium from the system becomes slowand when it is applied to the above-mentioned ink for ink jet recording,there are problems that the drying property of ink is lowered, thepigment is diffused in the inside of a printed article and printingdensity is also lowered.

Consequently, a method of coating the surface of a pigment with a filmforming resin to obtain a resin-coated pigment is proposed as a methodof improving the dispersion stability of the pigment and not causing thelowering of drying property and density of a printed article. Forexample, there are disclosed methods including a method of coating ananionic group-containing resin on the surface of a pigment by a phasetransfer emulsification method (JP 10-046075 A and JP 10-140065 A) and amethod of coating an anionic group-containing resin on the surface of apigment by an acid precipitation method (JP 10-140065 A and JP 09-031360A). In the phase transfer emulsification method, a dispersion in which apigment is dispersed in an organic solvent solution of an anionicgroup-containing resin neutralized with a basic compound is prepared,the dispersion is mixed with a large quantity of an aqueous medium, andthen, the organic solvent is removed if necessary, to obtain an aqueouspigment dispersion containing a resin-coated pigment. In the acidprecipitation method, a pigment is dispersed in an aqueous solution inwhich an anionic group-containing resin has been dissolved in thepresence of a basic compound, and then an acidic compound is added toadjust pH to neutral or acidic condition, thereby coating the surface ofthe pigment with the anionic group-containing resin to obtain a pigmentcake, and then, the resin-coated pigment is dispersed again using abasic compound, to obtain an aqueous pigment dispersion with gooddispersibility.

However, when the phase transfer emulsification method is used, theresin is really interacted with only an organic solvent in dispersing apigment; therefore the expected effect of dispersing the pigment is notobtained. Further, since a large quantity of organic solvent remains inthe system, storage stability is lowered and on the other hand, ifexcessive organic solvent is removed, there is a problem that it is noteconomically advantageous. When the acid precipitation method is used,the system contains the salt of acid and base which causes the loweringof various performances that are required for the ink for ink jetrecording, and there is a problem that adequate washing is required forremoving the salt.

DISCLOSURE OF INVENTION

It is a problem of the present invention to provide a resin-coatedpigment capable of providing an ink or a paint being superior indispersion stability of a pigment and having rapid drying property, highgloss and density without containing those such as an organic solventand a salt that cause the lowering of performance of pigment dispersion,an aqueous pigment dispersion using the same, and an aqueous colorantcomposition containing the aqueous pigment dispersion.

The present inventors have carried out studies for solving theabove-mentioned problem and as a result, have developed a quite novelprocess for producing a resin-coated pigment wherein a pigment isdispersed in an aqueous medium by using an anionic group-containingresin solubilized with a basic compound as a dispersant (this step isthe same as the step in a conventional process for producing an aqueouspigment dispersion), and then the basic compound in the aqueous mediumis removed using an ion exchange means to precipitate the anionicgroup-containing resin on the surface of the pigment (hereinafter, “toprecipitate a resin” on the surface of a pigment is occasionallyrepresented as “coated with a resin”). Further, a pigment dispersionobtained by neutralizing a portion or all of the anionic group of thethus obtained resin-coated pigment of the present invention using abasic compound and dispersing it into an aqueous medium (hereinafter,this dispersion treatment is occasionally called as re-dispersion) isextremely superior in dispersion stability of a pigment. Further, it hasbeen found out that the re-dispersion of the resin-coated pigmentcontains extremely little amount of a resin not adsorbed on the pigmentin the system, as compared with an aqueous pigment dispersion obtainedby using an anionic group-containing resin merely as a dispersant andfurther an aqueous pigment dispersion obtained by using the phasetransfer emulsification method or the acid precipitation method. As aresult, an aqueous colorant composition using the aqueous pigmentdispersion, in particular, an aqueous ink composition for ink jetrecording, is superior in storage stability, good in spouting stability,high in density and superior in drying property, vividness and gloss.

The reason is deduced as follows: In the method of precipitating ananionic group-containing resin on the surface of a pigment by using anion exchange means, an organic solvent and a salt do not intervene,which is advantageous for enhancing adsorption force between the pigmentand the resin, and this state can be kept even if it is perfectlyneutralized with a basic compound. Further, since the amount of a resinnot adsorbed on the pigment is little, high solid content and lowviscosity can be achieved and the permeation of the aqueous medium intoa printed article is accelerated and as a result drying property andcolor density can be also improved.

Further, it has been found in the present invention that when theremoval of a basic compound by ion exchange means is partially carriedout within a range not causing precipitation of the resin-coated pigmentin the process for producing the fore-mentioned resin-coated pigment, anaqueous pigment dispersion with good pigment dispersibility can bedirectly produced without requiring a step of re-dispersing theresin-coated pigment using the basic compound.

Thus, the present inventors have found that all of the problems of theabove-mentioned prior art can be solved by the process for producing aresin-coated pigment utilizing an ion exchange means and have completedthe present invention.

The present invention provides a process for producing a resin-coatedpigment, a resin-coated pigment, an aqueous pigment dispersion and anaqueous colorant composition as mentioned below.

(1) A process for producing a pigment coated with a resin, comprisingthe steps of:

dispersing a pigment into an aqueous solution in which an anionicgroup-containing resin is dissolved in the presence of a basic compound,and

then removing the basic compound in the aqueous solution by using an ionexchange means to precipitate the anionic group-containing resin on thesurface of the pigment.

(2) The process for producing a pigment coated with a resin of thefore-mentioned item (1), wherein the basic compound in the aqueoussolution is removed by replacing the cation of the basic compound by ahydrogen ion, using an organic ion exchanger and/or an inorganic ionexchanger as the ion exchange means.

(3) The process for producing a pigment coated with a resin of thefore-mentioned item (2), wherein substantially the whole amount of thebasic compound in the aqueous solution is removed.

(4) The process for producing a pigment coated with a resin of thefore-mentioned item (2), wherein the ion exchange is carried out untilthe pH of the aqueous solution is less than 4.

(5) The process for producing a pigment coated with a resin of any oneof the fore-mentioned items (1) to (4), wherein the acid value of theanionic group-containing resin is 40 to 300 KOH mg/g.

(6) A pigment coated with a resin produced by the process of any one ofthe fore-mentioned items (1) to (5).

(7) An aqueous pigment dispersion obtained by dispersing the pigmentcoated with a resin of the fore-mentioned item (6) in an aqueous mediumusing a basic compound.

(8) A process for producing an aqueous pigment dispersion, comprisingthe steps of:

dispersing a pigment into an aqueous solution in which an anionicgroup-containing resin is dissolved in the presence of a basic compound,and

then partially removing the basic compound in the aqueous solution byusing an ion exchange means.

(9) The process for producing an aqueous pigment dispersion of thefore-mentioned item (8), wherein the basic compound in the aqueoussolution is partially removed by replacing a portion of the cation ofthe basic compound by a hydrogen ion, using an organic ion exchangerand/or an inorganic ion exchanger as the ion exchange means.

(10) The process for producing an aqueous pigment dispersion of thefore-mentioned item (9), wherein the ion exchange is carried out untilthe pH of the aqueous solution is within a range of 4 to 8.

(11) The process for producing an aqueous pigment dispersion of any oneof the fore-mentioned items (8) to (10), wherein the acid value of theanionic group-containing resin is 40 to 300 KOH mg/g.

(12) An aqueous colorant composition comprising the aqueous pigmentdispersion of the fore-mentioned item (7).

(13) The aqueous colorant composition of the fore-mentioned item (12),which is an aqueous ink composition for ink jet recording.

(14) An aqueous colorant composition comprising the aqueous pigmentdispersion obtained by the process of any one of the fore-mentioneditems (8) to (11).

(15) The aqueous colorant composition of the fore-mentioned item (14),which is an aqueous ink composition for ink jet recording.

In accordance with the present invention, a resin-coated pigment isobtained by a novel process wherein a pigment is dispersed into anaqueous solution in which an anionic group-containing resin is dissolvedin the presence of a basic compound, and then, a portion or whole of thebasic compound in the aqueous solution is removed by using an ionexchange means to precipitate the anionic group-containing resin on thesurface of the pigment. An aqueous pigment dispersion containing theresin-coated pigment obtained by the process is superior in storagestability and provides coating film with high density, and superiorvividness and gloss.

BEST MODE FOR CARRYING OUT THE INVENTION

The process for producing a resin-coated pigment, the resin-coatedpigment, the aqueous pigment dispersion and the aqueous colorantcomposition in accordance with the present invention are specificallydescribed below.

Firstly, in the process for producing a resin-coated pigment inaccordance with the present invention, a pigment is added to an aqueoussolution in which an anionic group-containing resin is dissolved in thepresence of a basic compound (preferably, the pigment and the anionicgroup-containing resin are added so that the amount (on a solid basis)of the anionic group-containing resin is 10 to 100 parts by weigh basedon 100 parts by weight of the pigment), and the pigment is dispersed bya dispersing machine such as a mill type dispersing machine or a highpressure dispersing machine. Then, the resultant pigment dispersion,after being diluted if necessary, undergoes ion exchange of the cationof the basic compound with hydrogen ion by directly adding an organicion exchanger and/or an inorganic ion exchanger to the pigmentdispersion, or by passing the pigment dispersion through an ion exchangecolumn packed with such an ion exchanger, thereby precipitating(coating) the anionic group-containing resin on the surface of thepigment. Further, if necessary, filtration, washing with water anddrying are carried out to obtain a resin-coated pigment. Theresin-coated pigment can be used in a moisture-containing cake state, adry state or a state dispersed in an appropriate medium.

The resin-coated pigment obtained by the above-mentioned productionprocess is used for uses such as toner as a dry composition or for anaqueous pigment dispersion as uses of various wet compositions.

The pigment applicable for the process for producing a resin-coatedpigment in accordance with the present invention is not specificallylimited. Examples thereof include inorganic pigments such as carbonblack, titanium black, titanium white, zinc sulfide and iron oxide red,and organic pigments such as a quinacridone pigment, a dioxazinepigment, a phthalocyanine pigment, an anthrapyrimidine pigment, ananthanthrone pigment, an indanthrone pigment, a flavanthrone pigment, aperylene pigment, a diketopyrrolopyrrole pigment, a perinone pigment, aquinophthalone pigment, an anthraquinone pigment, a thioindigo pigment,a benzimidazolone pigment, an isoindolinone pigment, an azomethinepigment and an azo pigment.

As the anionic group including resin applicable for the process forproducing the resin-coated pigment of the present invention, any anionicgroup-containing resins that can be utilized for pigment dispersion oras binders for usual inks and paints can be used without particularlimitation as long as it can be dissolved in an aqueous medium in thepresence of a basic compound, but a resin containing one or at least twoof anionic groups such as a carboxyl group, a sulfonic acid group and aphosphono group (—P(═O)(OH)₂) that can be dissolved in an aqueous mediumin the presence of a basic compound is preferably used.

The acid value of the anionic group-containing resin is preferably 40 to300 KOH mg/g and more preferably 70 to 250 KOH mg/g. When the acid valueof the anionic group-containing resin is lower than the range, thedispersion stability of the aqueous dispersion of the resin-coatedpigment obtained is occasionally lowered and on the other hand, when itis more than the range, hydrophilic property is excessively high;therefore the storage stability and water resistance are occasionallylowered.

Further, the anionic group-containing resin preferably has a hydrophobicportion for mainly improving adsorbtion to a pigment and/or ahydrophilic portion for exhibiting action effective for prevention ofcoagulation in an aqueous medium, in a molecule. As the hydrophobicportion introduced in a molecule, hydrophobic groups such as a longchain alkyl group and an alicyclic or aromatic cyclic hydrocarbon groupare exemplified, and as the hydrophilic portion introduced in amolecule, hydrophilic groups such as a (poly)oxyalkylene chain, a basicnitrogen-containing group, a hydroxyl group and an epoxy group areexemplified.

The anionic group-containing resin preferably has a weight averagemolecular weight of 3,000 to 200,000 usually, more preferably 7,000 to100,000.

Typical examples of the anionic group-containing resin include anionicgroup-containing resins such as an acrylic copolymer resin obtained bycopolymerizing an anionic group-containing radically polymerizablemonomer with other radically polymerizable monomer, and a maleic acidbased copolymer resin, and anionic group-containing resins such as apolyester resin and a polyurethane resin that are obtained bycondensation polymerization reaction.

The materials for synthesizing the anionic group-containing resin aredisclosed in, for example, JP 2000-94825 A. Acrylic copolymer resins,maleic acid based copolymer resins, polyester resins and polyurethaneresins resin that are obtained by using the materials described in thepublication, can be utilized. Of course, resins obtained by using othermaterials can be also utilized.

As the anionic group-containing resin, for example, there can be usedthose that are obtained by polymerizing a mixture of an anionicgroup-containing monomer with other monomer that is suitably selected inaccordance with purpose for use and is copolymerizable with the anionicgroup-containing monomer, in the presence of a usual radical generatingagent (for example, such as benzoyl peroxide, tert-butylperoxy benzoateand azobisisobutyronitrile) in a solvent.

The anionic group-containing monomers include, for example, monomershaving at least one anionic group selected from the group consisting ofa carboxyl group, a sulfonic acid group and a phosphono group, and amongthese, a monomer having a carboxyl group is preferable in particular.

Examples of the monomer having a carboxyl group include acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid, 2-carboxyethyl (meth)acrylate, 2-carboxypropyl (meth)acrylate,maleic anhydride, fumaric anhydride and maleic acid half ester.

Examples of the monomer having a sulfonic acid group include sulfoethylmethacrylate.

Examples of the acryl monomer having a phosphono group includephosphonoethyl methacrylate.

Then, other monomers copolymerizable with the anionic group-containingmonomer are explained.

In order to introduce the hydrophobic portion or the hydrophilic portionin a molecule, a method of synthesizing a resin using a monomer havingsuch a portion can be utilized.

Examples of usable hydrophobic group-containing monomer include monomershaving a long chain alkyl group such as alkyl esters of radicallypolymerizable unsaturated carboxylic acid wherein the alkyl group has atleast 8 carbon atoms (e.g. 2-ethylhexyl (meth)acrylate, octyl(meth)acrylate, lauryl (meth)acrylate, stearyl (meth)acrylate, and2-hydroxystearyl (meth)acrylate), alkyl vinyl ethers wherein the alkylgroup has at least 8 carbon atoms (e.g. dodecyl vinyl ether), and vinylesters of fatty acid having at least 8 carbon atoms (e.g. vinyl2-ethylhexanoate, vinyl laurate, and vinyl stearate); monomers having analicyclic hydrocarbon group such as cyclohexyl (meth)acrylate; andmonomers having an aromatic hydrocarbon group (e.g. benzyl(meth)acrylate, styrene monomers such as styrene, α-styrene, and vinyltoluene). Examples of usable hydrophilic group-containing monomerinclude monomers having a (poly)oxyalkylene chain such as ester productsof (poly)alkylene glycols whose one terminal is blocked with an alkylgroup (e.g. methoxypolyethylene glycol,methoxypolyethylene-polypropylene glycol, ethoxypolyethylene glycol,ethoxypolyethylene-polypropylene glycol, propoxypolyethylene glycol andpropoxypolyethylene-polypropylene glycol) with radically polymerizableunsaturated carboxylic acids such as (meth)acrylic acid, and adducts ofethylene oxide and/or adducts of propylene oxide to radicallypolymerizable unsaturated carboxylic acid such as (meth)acrylic; basicgroup-containing monomers such as vinylpyrrolidones (e.g.1-vinyl-2-pyrrolidone and 1-vinyl-3-pyrrolidone), vinylpyridines (e.g.2-vinylpyridine, 4-vinylpyridine, 5-methyl-2-vinylpyridine and5-ethyl-2-vinylpyridine), vinylimidazoles (e.g. 1-vinylimidazole and1-vinyl-2-methylimidazole), vinylpiperidines (e.g. 3-vinylpiperidine andN-methyl-3-vinylpiperidine), and nitrogen-containing derivatives of(meth)acrylic acid (e.g. dimethylaminoethyl (meth)acrylate,diethylaminoethyl (meth)acrylate, tert-butylaminoethyl (meth)acrylate,(meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl(meth)acrylamide, N-methoxy (meth)acrylamide, N-ethoxy (meth)acrylamide,N-dimethyl acrylamide and N-propyl acrylamide); monomers having ahydroxyl group such as hydroxyalkyl esters of (meth)acrylic acid (e.g.hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate); andmonomers having an epoxy group such as glycidyl (meth)acrylate.

Examples of other monomers include alkyl esters of (meth)acrylic acidwherein the alkyl group has less than 8 carbon atoms, such as methyl(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl(meth)acrylate, butyl (meth)acrylate and hexyl (meth)acrylate.

The basic compounds used for dissolving an anionic group-containingresin in an aqueous medium include inorganic basic compounds such assodium hydroxide and potassium hydroxide, and organic basic compoundssuch as ammonia, methylamine, ethylamine, monoethanolamine,N,N-dimethylethanolamine, N,N-diethylethanolamine,N,N-dibutylethanolamine, diethanolamine, N-methyldiethanolamine,triethanolamine, morpholine, N-methylmorpholine and N-ethylmorpholine.These basic compounds can be used alone or in combination of two or morespecies thereof. Among these, alkanol amines such as monoethanolamine,N,N-dimethylethanolamine, N,N-diethylethanolamine,N,N-dibutylethanolamine, diethanolamine, N-methyldiethanolamine andtriethanolamine are preferable. The amount of the basic compound usedmay be an amount at which the anionic group-containing resin can bedissolved in an aqueous medium and is usually within a range of 80 to120% of the quantity required for neutralization. Of course, the basiccompound can be used at an amount exceeding the range.

As the aqueous medium for dissolving the anionic group-containing resin,water or a mixture of water and a water-miscible organic solvent can beused. Examples of the water-miscible organic solvent include loweralcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol andn-propyl alcohol; polyvalent alcohols such as ethylene glycol, propyleneglycol, diethylene glycol, dipropylene glycol and glycerin; monoalkylethers of (poly)alkylene glycol such as ethylene glycol monomethylether, ethylene glycol monoethyl ether, propylene glycol monomethylether, propylene glycol monoethyl ether, diethylene glycol monomethylether and diethylene glycol monoethyl ether; and monofatty acid estersof (poly)alkylene glycol such as ethylene glycol monoacetate andpropylene glycol monoacetate. These water-miscible organic solvents canbe used alone or in combination of two or more species thereof.

As the ion exchange means that can be applied to the process forproducing a resin-coated pigment in accordance with the presentinvention, an organic ion exchanger, an inorganic ion exchanger and acombination thereof can be used. As the organic ion exchanger and theinorganic ion exchanger, any organic or inorganic ion exchanger may beused as long as it has ion exchange ability for replacing the cation ofthe basic compound by hydrogen may be used, and for example, ionexchange resins and inorganic ion exchangers can be used. Morespecifically, the ion exchange resins include those having ion exchangegroup such as a carboxyl group, a sulfonic acid group or a phosphonogroup on resin substrates such as styrene resin, acrylic resin, phenolresin, aliphatic hydrocarbon resin and pyridine based resin; and theinorganic ion exchangers include inorganic laminar clay minerals andzeolite.

In one embodiment of the process for producing a resin-coated pigment inaccordance with the present invention, when the basic compound in thedispersion obtained by dispersing a pigment in an aqueous solution inwhich an anionic group-containing resin is dissolved in the presence ofthe basic compound is removed by ion exchange, the basic compoundexisting in the system is removed until a resin-coated pigment isprecipitated or the system becomes a paste-like state (hereat, thepaste-like state means a state in which a resin-coated pigment isprecipitated when the paste is diluted with an aqueous medium).Specifically, for example, the ion exchange is carried out until the pHof the system is less than about 4 or all of the basic compound existingin the system is substantially removed. Under the condition, nearly allamount of the anionic group-containing resin in the system can beprecipitated on the surface of the pigment, so that a resin-coatedpigment is precipitated or the system becomes a paste-like state. Theresultant precipitate is filtered off, washed with water and ifnecessary, dried to give a resin-coated pigment. The resin-coatedpigment can be used in a state of moisture-containing cake or in a drystate.

Then, in the process for producing an aqueous pigment dispersion inaccordance with the present invention, a portion or all of the anionicgroup-containing resin in the resin-coated pigment produced as mentionedabove is neutralized with a basic compound and the resultant isdispersed again in an aqueous medium using any of various dispersingmachines to give an aqueous pigment dispersion superior in dispersionstability. The amount of the basic compound used is preferably 10 to300%, more preferably 50 to 120%, based on the acid value of the anionicgroup of the anionic group-containing resin. When the solidconcentration of the aqueous pigment dispersion obtained is low, thesolid concentration is heightened by a method such as ultrafiltration.The content (on a solid basis) of the resin-coated pigment contained inthe aqueous pigment dispersion obtained is preferably 70% by weight orless, more preferably 2 to 60% by weight, still more preferably 10 to50% by weight, based on the total amount of the aqueous pigmentdispersion.

The aqueous medium and the basic compound used for preparing the aqueouspigment dispersion are suitably selected from the materials used in theprocess for producing the resin-coated pigment in accordance with theiruse.

In another embodiment of the process for producing an aqueous pigmentdispersion in accordance with the present invention, when the basiccompound in the dispersion obtained by dispersing a pigment in anaqueous solution in which an anionic group-containing resin is dissolvedin the presence of the basic compound is removed by ion exchange in theabove-mentioned process for producing the resin-coated pigment, thebasic compound existing in the system is partially removed within arange not causing precipitation of the resin-coated pigment. The rangenot causing precipitation of the resin-coated pigment varies dependingon the hydrophobic degree or hydrophilic degree of the anionicgroup-containing resin, the weight average molecular weight of theanionic group-containing resin and the kind of the aqueous medium used,but the ion exchange is preferably carried out until the pH of thedispersion is about 4 to 8. Since a portion of the anionic groups of theanionic group-containing resin precipitated on the surface of thepigment is neutralized by the basic compound and ionized under thecondition, a resin-coated pigment prepared is superior indispersibility. When this process for producing the aqueous pigmentdispersion in accordance with the present invention is used, an aqueouspigment dispersion superior in pigment dispersibility can be directlyobtained without the step of re-dispersion of the resin-coated pigmentonce produced. The basic compound used in the embodiment is suitablyselected from the basic compounds used in the process for producing theresin-coated pigment, considering pigment dispersibility and the use ofthe objective product.

The aqueous pigment dispersion of the present invention, in which aresin, various additives and water-soluble solvent are added ifnecessary, is used for uses such as printing inks, paints, inks forwriting instruments, inks for ink jet recording and liquid cosmetics. Inparticular, it is the aqueous pigment dispersion having extremelysuperior effect in a field requiring satisfactory pigment dispersibilityand dispersion stability, rapid drying property, high gloss and highdensity.

Then, an aqueous ink composition for ink jet recording containing theaqueous pigment dispersion is described.

The aqueous ink composition for ink jet recording in accordance with thepresent invention can be produced by adding a water-soluble resin, asurfactant, an antiseptic, a viscosity regulator, a pH adjuster and achelating agent if necessary in addition to the aqueous pigmentdispersion, a water-miscible organic solvent and water and mixing themixture with a simple agitating apparatus such as a Disper.

The content of the aqueous pigment dispersion in the aqueous inkcomposition for ink jet recording in accordance with the presentinvention is preferably 1 to 100% by weight, more preferably 5 to 100%by weight, considering the color density, fine definition, transparencyand chromaticness required for the aqueous ink composition for ink jetrecording.

As the water-miscible solvent used for producing the aqueous inkcomposition for ink jet recording in accordance with the presentinvention, the same solvent as those used for obtaining the resin-coatedpigment and the aqueous pigment dispersion can be used and among these,polyvalent alcohols and ethers are preferable.

EXAMPLES

The present invention is specifically described below on the basis ofExamples, but the present invention is not limited to these Examples.Hereinafter, parts and % mean parts by weight and % by weightrespectively unless otherwise noticed.

<Preparation Example of Anionic Group-Containing Resin> [AnionicGroup-Containing Resin 1]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 7.7 parts of methacrylicacid, 21.8 parts of methyl methacrylate, 70.5 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 1 with aweight average molecular weight of 11,000 and an acid value of 50 mgKOH/g.

KAYA ESTER O-50TL: 50% toluene solution of t-butylperoxy2-ethylhexanoate represented by the under-mentioned formula.

CH₃—C(CH₃)₂—O—O—C(═O)—CH(CH₂CH₃)—(CH₂)₃—CH₃

[Anionic Group-Containing Resin 2]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 12.3 parts of methacrylicacid, 20.7 parts of methyl methacrylate, 67.0 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 2 with aweight average molecular weight of 10,000 and an acid value of 80 mgKOH/g.

[Anionic Group-Containing Resin 3]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 18.4 parts of methacrylicacid, 19.3 parts of methyl methacrylate, 62.3 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 3 with aweight average molecular weight of 10,000 and an acid value of 120 mgKOH/g.

[Anionic Group-Containing Resin 4]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 22.3 parts of methacrylicacid, 18.3 parts of methyl methacrylate, 59.4 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 4 with aweight average molecular weight of 12,000 and an acid value of 145 mgKOH/g.

[Anionic Group-Containing Resin 5]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 24.5 parts of methacrylicacid, 17.9 parts of methyl methacrylate, 57.6 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 5 with aweight average molecular weight of 12,000 and an acid value of 160 mgKOH/g.

[Anionic Group-Containing Resin 6]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 30.6 parts of methacrylicacid, 16.4 parts of methyl methacrylate, 53.0 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 6 with aweight average molecular weight of 11,000 and an acid value of 200 mgKOH/g.

[Anionic Group-Containing Resin 7]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 36.7 parts of methacrylicacid, 14.9 parts of methyl methacrylate, 48.4 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 7 with aweight average molecular weight of 12,000 and an acid value of 240 mgKOH/g.

[Anionic Group-Containing Resin 8]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 42.8 parts of methacrylicacid, 13.5 parts of methyl methacrylate, 43.7 parts of styrene, 7.2parts of KAYA ESTER O-50TL as an initiator and 46 parts of butyl acetatewas added dropwise over 1.5 hours while introducing nitrogen gas, andthe polymerization was further carried out for 2 hours while keeping themixture at the same temperature. The solvent was distilled off underreduced pressure to obtain anionic group-containing resin 8 with aweight average molecular weight of 10,000 and an acid value of 280 mgKOH/g.

[Anionic Group-Containing Resin 9]

Into a four necked flask equipped with a stirrer, a condenser and anitrogen gas introducing pipe was placed 180 parts of butyl acetate, andthe content was heated to 100° C. A mixture of 30.0 parts of butylmaleate, 11.9 parts of methacrylic acid, 13.7 parts of methylmethacrylate, 44.4 parts of styrene, 7.2 parts of KAYA ESTER O-50TL asan initiator and 46 parts of butyl acetate was added dropwise over 1.5hours while introducing nitrogen gas, and the polymerization was furthercarried out for 2 hours while keeping the mixture at the sametemperature. The solvent was distilled off under reduced pressure toobtain anionic group-containing resin 9 with a weight average molecularweight of 10,000 and an acid value of 192 mg KOH/g.

<Preparation Example of Anionic Group-Containing Resin Solution>

Each of anionic group-containing resins 1 to 9 was dissolved into amixture of water and N,N-dimethylethanolamine whose amount was thatrequired for neutralizing 100% of the acid value of each resin, toobtain each of anionic group-containing resin solutions 1 to 9 with asolid content of 30%.

<Examples Relating to Resin-Coated Pigment> [Pigment Dispersion]

71.5 parts of water was added to 13.5 parts of each of anionicgroup-containing resin solutions 1 to 9. 15 parts of a pigment (PRINTEX80, manufactured by Degussa Corporation, carbon black) was added to themixture and mixed with stirring. The resultant was kneaded with a wetcirculation mill to give each of pigment dispersions 1 to 9.

[Resin-Coated Pigment]

Each of pigment dispersions 1 to 9 was diluted with water so that thepigment concentration was 5%. A cation exchange resin (DOWEX MONOSPHERE650C (H)650C manufactured by Dow Chemical Co., Ltd.) was added to thediluted solution in an amount of 5% based on the diluted solution andstirred, and the ion exchange was carried out until the pH was less than4 to obtain each of resin-coated pigments 1 to 9. Then, the ion exchangeresin was filtered off with a mesh and a moisture-containing cake(pigment concentration: 15%) containing each of resin-coated pigments 1to 9 was obtained by means of suction filtration.

<Examples Relating to Preparation of Aqueous Pigment Dispersion byRe-Dispersion of Resin-Coated Pigment> Aqueous Pigment Dispersions ofExamples 1 to 9

Triethylenediamine whose amount was that required for neutralizing 80%of the acid value of the resin in each of resin-coated pigments 1 to 9and water whose amount was that required for adjusting the pigmentconcentration to 10% were added to each of moisture-containing cakescontaining resin-coated pigments 1 to 9, and the mixture was stirredwith a Disper to obtain each of aqueous pigment dispersions of Examples1 to 9.

Aqueous Pigment Dispersions of Comparative Examples 1 to 9

71.5 parts of water was added to 13.5 parts of each of anionicgroup-containing resin solutions that were obtained by changingN,N-dimethylethanolamine in anionic group-containing resin solutions 1to 9 to triethylenediamine. 15 parts of a pigment (PRINTEX 80manufactured by Degussa Corporation, carbon black) was added to themixture and mixed with stirring. The resultant was kneaded with a wetcirculation mill and then was diluted with water so that the pigmentconcentration was 10%, to obtain each of aqueous pigment dispersions ofComparative Examples 1 to 9.

Ink Compositions for Ink Jet Recording of Examples 1 to 9 andComparative Examples 1 to 9

39 parts of water, 15 parts of glycerin, 1 part of acetylene glycol and5 parts of butyl triglycol (triethylene glycol monobutyl ether) weremixed with stirring to 40 parts of each of aqueous pigment dispersionsof Examples 1 to 9 and Comparative Examples 1 to 9, to obtain each ofaqueous pigment type ink compositions for ink jet recording of Examples1 to 9 and Comparative Examples 1 to 9.

<Performance Evaluation of Aqueous Pigment Dispersions and AqueousPigment Type Ink Compositions for Ink Jet Recording>

They were evaluated by the evaluation methods below and the results areshown in Tables 1 and 2.

[Storage Stability]

The viscosities at 25° C. of the aqueous pigment dispersions and theaqueous pigment type ink compositions for ink jet recording of Examples1 to 9 and Comparative Examples 1 to 9 and viscosities at 25° C. afterstorage for 10 days at 60° C. were measured. The storage stability wasevaluated in terms of rise rate of viscosity.

Evaluation Criteria

A: The rise rate of viscosity is less than 1.1-fold.

B: The rise rate of viscosity is 1.1 to 1.5-fold.

C: The rise rate of viscosity is more than 1.5-fold.

[Performance Evaluation of Printed Article]

The aqueous pigment type ink compositions for ink jet recording ofExamples 1 to 9 and Comparative Examples 1 to 9 were printed on plainpaper (Xerox L manufactured by Xerox Corporation) and dedicated glosspaper (PM photo paper, manufactured by Seiko Epson Corporation) using acommercially available ink jet printer (EM-900C, piezo type,manufactured by Seiko Epson Corporation). The printed articles obtainedwere evaluated by the evaluation methods below.

Printing Density

The density of solid portion of printed articles was measured with aMacbeth optical densitometer RT-918 (manufactured by Gretag Macbeth Co.)and evaluated on the basis of the following criteria.

Plain Paper

A: The density is more than 1.20.

B: The density is more than 1.10 and not more than 1.20.

C: The density is not more than 1.10.

Dedicated Paper

A: The density is more than 2.00.

B: The density is more than 1.80 and not more than 2.00.

C: The density is not more than 1.80.

Gloss

The gloss of solid portion of the printed articles in which the inkcompositions were printed on the dedicated paper was measured with agloss meter (PG-1M manufactured by Nippon Denshoku Industries Co.,Ltd.). The gloss was measured with a regular reflection of 60° andevaluated on the basis of the following criteria.

A: The gloss is not less than 90.

B: The gloss is not less than 80 and less than 90.

C: The gloss is less than 80.

TABLE 1 (Evaluation of aqueous pigment dispersions) Ex. Com. Ex. 1 2 3 45 6 7 8 9 1 2 3 4 5 6 7 8 9 Acid value of resin 50 80 120 145 160 200240 280 192 50 80 120 145 160 200 240 280 192 Storage stability B A A AA A A A A C B B B B B B C B

TABLE 2 (Evaluation of ink compositions for ink jet recording) Ex. Com.Ex. 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Acid value of resin 50 80 120145 160 200 240 280 192 50 80 120 145 160 200 240 280 192 Storagestability A A A A A A A A A C C B B B B C C C Printing density of A A AA A B B B B B B B C C C C C C plain paper Printing density of B A A A AA A A A C B B B B B B B B dedicated paper Gloss of dedicated B B B A A AA A A C C C C B B A A B paper

Aqueous Pigment Dispersions of Examples 10 to 13

Each of pigment dispersions 2, 4, 6 and 8 was diluted with water so thatthe pigment concentration was 10%. A cation exchange resin (DOWEXMONOSPHERE 650C (H)650C manufactured by Dow Chemical Co., Ltd.) wasadded to the diluted solution in an amount of 5% based on the dilutedsolution and stirred, and the ion exchange was carried out until the pHwas 6.5. Then, the ion exchange resin was filtered off with a mesh toobtain each of aqueous pigment dispersions (pigment concentration: 10%)of Examples 10 to 13.

Aqueous Pigment Dispersions of Comparative Examples 10 to 13

Pigment dispersions 2, 4, 6 and 8 were diluted with water so that thepigment concentration was 10% to obtain aqueous pigment dispersions ofComparative Examples 10 to 13, respectively.

Ink Compositions for Ink Jet Recording of Examples 10 to 13 andComparative Examples 10 to 13

39 parts of water, 15 parts of glycerin, 1 part of acetylene glycol and5 parts of butyl triglycol (triethylene glycol monobutyl ether) weremixed with stirring to 40 parts of each of aqueous pigment dispersionsof Examples 10 to 13 and Comparative Examples 10 to 13 to obtain each ofaqueous pigment type ink compositions for ink jet recording of Examples10 to 13 and Comparative Examples 10 to 13.

<Performance Evaluation of Aqueous Pigment Dispersions and AqueousPigment Type Ink Compositions for Ink Jet Recording>

The aqueous pigment dispersions of Examples 10 to 13 and ComparativeExamples 10 to 13 and the ink compositions for ink jet recording ofExamples 10 to 13 and Comparative Examples 10 to 13 were evaluated forstorage stability by the same method as mentioned above. The printedarticles obtained by using the ink compositions for ink jet recording ofExamples 10 to 13 and Comparative Examples 10 to 13 were evaluated bythe same method as mentioned above. The results are shown in Tables 3and 4.

TABLE 3 (Evaluation of aqueous pigment dispersions) Ex. Com. Ex. 10 1112 13 10 11 12 13 Acid value of 80 145 200 280 80 145 200 280 resinStorage stability A A A A B B B C

TABLE 4 (Evaluation of ink compositions for ink jet recording) Ex. Com.Ex. 10 11 12 13 10 11 12 13 Acid value of resin 80 145 200 280 80 145200 280 Storage stability A A A A C B B C Printing density of A A A B BC C C plain paper Printing density of A A A A B C C C dedicated paperGloss of dedicated B A A A C C B A paper

1: A process for producing a pigment coated with a resin, comprising thesteps of: dispersing a pigment into an aqueous solution in which ananionic group-containing resin is dissolved in the presence of a basiccompound, and then removing the basic compound in the aqueous solutionby using an ion exchange means to precipitate the anionicgroup-containing resin on the surface of the pigment. 2: The process forproducing a pigment coated with a resin of claim 1, wherein the basiccompound in the aqueous solution is removed by replacing the cation ofthe basic compound by a hydrogen ion, using an organic ion exchangerand/or an inorganic ion exchanger as the ion exchange means. 3: Theprocess for producing a pigment coated with a resin of claim 2, whereinsubstantially the whole amount of the basic compound in the aqueoussolution is removed. 4: The process for producing a pigment coated witha resin of claim 2, wherein the ion exchange is carried out until the pHof the aqueous solution is less than
 4. 5: The process for producing apigment coated with a resin of claim 1, wherein the acid value of theanionic group-containing resin is 40 to 300 KOH mg/g. 6: A pigmentcoated with a resin produced by the process of claim
 1. 7: An aqueouspigment dispersion obtained by dispersing the pigment coated with aresin of claim 6 in an aqueous medium using a basic compound. 8: Aprocess for producing an aqueous pigment dispersion, comprising thesteps of: dispersing a pigment into an aqueous solution in which ananionic group-containing resin is dissolved in the presence of a basiccompound, and then partially removing the basic compound in the aqueoussolution by using an ion exchange means. 9: The process for producing anaqueous pigment dispersion of claim 8, wherein the basic compound in theaqueous solution is partially removed by replacing a portion of thecation of the basic compound by a hydrogen ion, using an organic ionexchanger and/or an inorganic ion exchanger as the ion exchange means.10: The process for producing an aqueous pigment dispersion of claim 9,wherein the ion exchange is carried out until the pH of the aqueoussolution is within a range of 4 to
 8. 11: The process for producing anaqueous pigment dispersion of claim 8, wherein the acid value of theanionic group-containing resin is 40 to 300 KOH mg/g. 12: An aqueouscolorant composition comprising the aqueous pigment dispersion of claim7. 13: The aqueous colorant composition of claim 12, which is an aqueousink composition for ink jet recording. 14: An aqueous colorantcomposition comprising the aqueous pigment dispersion obtained by theprocess of claim
 8. 15: The aqueous colorant composition of claim 14,which is an aqueous ink composition for ink jet recording.